They could have flagged his visa -- twice. "U.S. Failed to Catch Suspect's Active Visa: Exclusive: 2 State Dept. Checks Before Alleged Terror Attack Didn't Discover Abdulmutallab had Active Visa Allowing Entry," by Armen Keteyian for CBS News, December 28 (thanks to Benedict):
CBS News has learned the State Department system designed to keep track of active U.S. visas twice failed to reveal Nigerian terror suspect Umar Farouk Abdulmutallab had been issued an active visa allowing him multiple entries into the United States.
According to a law enforcement source, the first failure came on Nov. 19, 2009, the very same day Abdulmutallab father's, Dr. Umaru Mutallab, a prominent banking official in Nigeria, expressed deep concern to officials at the U.S. Embassy in Abjua, Nigeria, that his 23-year-old son had fallen under the influence of "religious extremists" in Yemen.
The second failure to flag an active visa belonging to Abdulmuttalab occurred the very next day in Washington, after Mutallab's concerns were forwarded to officials there. It was only after the Christmas Day terror attack in Detroit that U.S. officials learned that Abdulmuttalab had been issued a visa by the U.S. Embassy in London valid from June 16, 2008, through June 12, 2010.